Positron beam lithography

ABSTRACT

Resist (402) is exposed by a beam of positrons (320) is an apparatus (300) similar to an electron beam lithography machine.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fabrication of items with smallfeatures, and, more paticularly, to lithographic methods of fabricationsuch as are used in semiconductor integrated circuits.

2. Description of the Related Art

Semiconductor-based electronic integrated circuits have progressivelyshrunk in feature size and increased in complexity since their inventionin 1958, and currently mass-produced integrated circuits such ascomputer memory chips have feature sizes on the order of 1 μm. Suchintegrated circuits are typically fabricated using photolithographywhich patterns a layer of photoresist (a radiation sensitive material)on the in-process integrated circuit by exposure of the photoresist tomasked radiation such as visible light or ultraviolet light. Theradiation causes a chemical change in the photoresist, and the exposedareas of the photoresist may then be selectively removed (positivephotoresist) or retained (negative photoresist) by contact with asolvent. The patterned photoresist is then used as a mask in a step(such as etching, deposition, ion implantation, etc.) of the fabricationprocess for the integrated circuit.

FIGS. 1a-c illustrate in cross sectional elevation views the formationand use of photoresist as an implantation mask. As shown in FIG. 1aphotoresist (positive) 102 is applied to substrate 104, photoresist 102is typically 1 μm thick. Next, ultraviolet radiation 106 is passedthrough pattern mask 108 and exposes photoresist 102 in the pattern ofmask 108; see FIG. 1b. The exposed portion of photoresist 102 is thenremoved (photoresist 102 is developed) by dissolution in a solvent, andthe developed photoresist 102 may then be used as a mask for ionimplantation of dopants 110 to form doped regions 112 in substrate 104;see FIG. 1c.

The feature size obtainable using photoresist lithography is limited bydiffraction effects, with resulting dimensions having a lower limit ofabout 0.1 μm even if ultraviolet light is used for the photoresistexposure. That is, in the exposure step illustrated in FIG. 1b, thewavelength of light 106 is comparable to the size of the openings inmask 108 and the light passing through the openings is severelydiffracted. In contrast, the use of resists sensitive to shortwavelength entities such as electrons, ions, or x-rays will eliminatethis diffraction limitation, but then the limitations of the resistmaterial itself become important. For example, electron beams can befocussed down to a spot size of the order of 10 Å, and can be used todirectly write on (expose) a resist layer without the use of a patternmask. The typical electron beam resist, polymethylmethacrylate (PMMA),is exposed by the incident electrons breaking bonds (e.g., carbon-carbonbonds) to make the PMMA more soluble in a developer such asmethylisobutylketone (MIBK). A spot size of less than 10 Å for theincident electron beam can be formed and accurately controlled withdigital and analog techniques; however, the incident electrons typicallyhave energies on the order of 20 KeV and create secondary electrons asthey inelastically scatter in the PMMA. These secondary electrons haveenergies up to about 100 eV and can also break carbon-carbon bonds. Thesecondary electrons have a range of up to about 100 Å from the incidentflux, and these account for the feature size limitation in PMMA of about125 Å. Molecular size and statistical effects may also limit featuresize. See R. Howard et al, Nanometer-Scale Fabrication Techniques in 5VLSI Electronics: Microstructure Science, pp. 150-153 (Academic Press1982). FIGS. 2a-b are cross sectional elevation views of Monte Carlosimulations that illustrate the spreading of the incident electrons andthe secondary electrons in a layer of PMMA.

Statistical effects due to fluctuations in the flux of incidentelectrons and fluctuations in the number of secondary electrons alsoaffect the feature size limitation for electron beam direct writeexposure of resist. Further, the incident electrons may also scatter offof the substrate back into the resist and further expose the resist. Theoverall result is a difficulty in achieving feature sizes approaching100 Å.

Attempts to overcome the feature size limitations of standardlithography include use of thin resist layers (to avoid secondaryelectron spread as in FIG. 2b), thin substrates (to avoid backscatteringof incident electrons), multilayer resists (effectively thin resist pluslower layers absorb backscatter). However, these approaches are not yetuseful in high volume manufacturing.

J. Cleaver et al, A Combined Electron and Ion Beam Lithography System, 3J. Vac. Sci. Tech. B 144 (1985) theoretically analyze a combination ofion beams and electron beams in a single machine for microlithography.

SUMMARY OF THE INVENTION

The present invention provides for the use of antiparticles in resistexposure. This solves the problems of the known electron beam methods ofresist exposure for small feature size lithography by reducing secondaryelectron creation and backscattering of incident electrons: theantiparticles are annihilated by electrons or nucleons in the resistyielding gamma rays that have only limited interaction with the resist.Preferred embodiments include use of an electron beam lithographymachine but with a positron source (such as Na²²) in place of theelectron gun. Other preferred embodiments use a positron beam to form alatent image by change of resist sensitivity to electrons, so subsequentexposure to an electron beam develops the latent image.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are schematic for clarity.

FIGS. 1a-c are cross sectional elevation views of prior art lithography;

FIGS. 2a-b illustrate scattering of incident electrons and secondaryelectrons during prior art electron beam resist exposure;

FIG. 3 is a schematic cross sectional elevation view of a firstpreferred embodiment apparatus; and

FIG. 4 illustrates first preferred embodiment resist exposure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a schematic cross sectional elevation view of a firstpreferred embodiment apparatus, generally denoted 300, for using thefirst preferred embodiment method of electron beam lithography.Apparatus 300 is essentially a standard electron beam lithographymachine but with a positron source in place of the typical electron gunand typically would include multiple lenses rather than the single lensillustrated. Apparatus 300 includes a sodium isotope (Na²²) source ofpositrons 302, a beam blanker 304 (which just electrostatically deflectsthe beam into a physical stop), deflection coils 306 which provide thescanning of the beam across the target substrate, electrostatic andmagnetic lenses 308 for focussing the beam of positrons to a spot on thetarget substrate, movable table 310 for holding the substrate,mechanical drive 312 for positioning table 310, position monitor 314 fordetecting the position of table 310, pattern data memory 318 which isinput to computer 316 for controlling beam blanker 304, deflection coils306, lenses 308, and mechanical drive 312. Positron beam 320 and targetsubstrate are held within vacuum chamber 322.

Positron source 302 includes a small block of sodium chloride with aportion of the sodium of isotope 22 (Na²²). The block is suspended in avacuum chamber and emits about 10⁹ positrons per second from the decayof the Na²² into Ne²². The positrons are emitted isotropically withenergies on the order of 1 MeV, and about 5-10% are electrostaticallycollected and slowed down to about 1 eV to form positron beam 320. These1 eV positrons constitute a current of about 10 pA. Alternatively, thepositrons collected can be slowed to energies of about 20 KeV to formpositron beam 320, and then immediately prior to beam 320 impingingresist 402 a retarding screen can drop the energy to about 1 eV. Ahigher energy beam 320 results in less Coulombic spreading due to theshorter time of flight, but a low beam current (10 pA as opposed totypical 1 nA electron beams) implies low charge density and minimalCoulombic spreading so high kinetic energies are not necessary.

The first preferred embodiment method of lithography proceeds asfollows:

(a) Coat substrate 400 with PMMA (polymethylmethacrylate) 402 to athickness of 1 μm; this coating may be obtained by spinning PMMA 402onto substrate 400. Note that substrate 400 may be a semiconductor suchas gallium arsenide (GaAs) or silicon already coated with layers ofdielectrics, semiconductors, and conductors such as metal layer 401, andPMMA 402 is spun onto the top layer. Insert coated substrate 400 ontotable 310 in apparatus 300. The pattern for PMMA 402 exposure is storedin memory 318 of apparatus 300.

(b) Activate positron source 302 to generate positron beam 320 andraster scan positron beam 320 across PMMA 402 on coated substrate 400 bydeflection coils 306 with the intensity of positron beam 320 switchedbetween zero and 10 pA by beam blanker 304. Lenses 308 focus beam 320 toa spot size of 10 Å at the top of PMMA 402. The positrons in beam 320have an average energy of about 1 eV, and upon penetrating PMMA 402break chemical bonds (including the carbon-carbon polymerization bonds)due to their annihilation of electrons constituting the chemical bonds.The scanning rate is set so that the 10 pA beam provides a sufficientdose of positrons to expose PMMA 402, and positron beam 320 iseffectively interrupted to achiveve pattern definition. The inelasticscattering of the positrons produces secondary electrons analogous tothe electrons in an electron beam inelastically scattering; however, thelow energy of the positrons implies little inelastic scattering becausethe bonding energy of electrons in PMMA 402 is comparable to the kineticenergy of the positrons and any secondary electrons will have negligibleenergy and not significantly break chemical bonds. In effect, low energypositrons generate many fewer secondary electrons than a typicalelectron beam but are just as or more effective in breaking chemicalbonds because the cross section for annihilation is large and theoffending chemical bond is literally removed. Because annihilationproducts (gamma rays) only weakly interact with PMMA 402, low energypositron exposure of PMMA 402 does not have the spread of electron beamexposure; see schematic illustration in FIG. 4 showing positronsinelastically scattering to generate secondary electrons andannihilating. Thus the more efficient bond breaking of positrons permitsa low energy beam and suppression of secondary electrons and exposurebroadening.

Because the resist chemical bond electron is truly destroyed rather thanjust scattered, regeneration of the bond is unlikely. Thus resists thatotherwise suffer from a loss of definition through bond reformation maybe used with positron beam exposure.

If positron beam 320 were of the same energy (20 KeV) as a typicalelectron beam, then the inelastic scattering and secondary electrongeneration within PMMA 402 would be comparable to that of an electronbeam. However, even high energy positron beams have advantages overelectron beams for resist exposure: A typical electron beam penetratesPMMA 402 and scatters off of substrate 400 (or metal layer 401) andgenerates large numbers of secondary electrons. Contrarily, a positronbeam penetrating PMMA 402 will have a large annihilation cross sectionin substrate 400 (or especially in metal layer 401) and not generate asmany secondary electrons. Further, a fraction of the positrons will beannihilated within PMMA 402 and not generate as many secondary electronsas a comparable electron.

(c) Remove coated substrate 400 with PMMA 402 from chamber 322, anddevelop the exposed PMMA 402 with a mixture of methyl isobutyl ketoneand isopropyl alcohol; that is, PMMA 402 is a positive resist with theexposed portion (broken bonds) removed during development. Then use thethus patterned PMMA 402 as a mask for ion implantation, etching, orlift-off deposition in the standard manner.

Second preferred embodiment method of lithography is similar to thefirst preferred embodiment and uses apparatus 300; however, rather thanraster scan positron beam 320 at a rate that will expose PMMA 402 on asingle pass, the scan rate is increased so that multiple (e.g., five orten) passes over the same area are required to expose PMMA 402. Thus thesecond preferred embodiment method averages out the statisticalfluctuations in apparatus 300 but does not require increased exposuretime because the scan rate is higher.

Third preferred embodiment method of lithography also uses apparatus300, but prior to positron beam exposure the method partially exposesPMMA 402 by a flood of electrons. This flood of electrons does not breaka sufficient number of chemical bonds to expose PMMA 402; however, asignificant fraction (e.g., 30%) of an exposure is achieved so positronbeam 320 has fewer chemical bonds to break to achieve full exposure andmay operate at higher scan rates or lower beam currents or both.

Fourth preferred embodiment method of lithography also uses apparatus300, but with a negative resist. (PMMA can also be used as a negativeresist but typically requires a much larger dose for exposure: PMMA as anegative resist is exposed by the incident electrons or positronsdisrupting chemical bonds and inducing formation of cross-linking bondsto increase molecular size and decrease solubility.) The method forms alatent image in the negative resist with positron beam 320 byannihilating electrons that would participate in the cross-linkingbonding. Subsequently, a flood electron exposure will form cross-linkingbonds in the complement of the latent image, and development will removethe resist at the latent image location. In effect, the negative resisthas been used as a positive resist, but with the latent image formed bylow energy positrons to avoid undesired exposure caused otherwise bysecondary electron spreading.

Fifth preferred embodiment lithography method and apparatus is similarto the first preferred embodiments but without resist and with heavierannihilative particles. In particular, the fifth preferred embodimentuses a source of negative pi mesons (or antiprotons or other stronglyinteracting particles that will not be Coulombically repelled from thesubstrate nuclei) in place of positron source 302 (this source could bea proton accelerator plus target) and relies on the strong interactionenergy released to directly etch a pattern in substrate 400 or metalfilm 401. Analogously, antineutrons could be used to directly etchsubstrate 400 or metal film 401, but the patterning would be with a maskbecause the neutral beam could not be focussed with electrostatic lensesand the magnetic dipole moment of neutrons is not useful for magneticlenses. Of course, the mask material will also be etched. Indeed, any ofthe beam exposure methods described supra could also be operated insteadwith a mask plus blanket exposure.

MODIFICATIONS AND ADVANTAGES

Various modifications of the preferred embodiment devices and methodsmay be made while retaining the features of annihilative matter orstrongly interacting matter as an exposing or etching agent.

For example, the types and dimensions and shapes and thicknesses of thesubstrates and resist layers could be varied, the beam of particlescould interact with substrate atoms to form dopants, and the embodimentsrequiring two different particle types could be used in an apparatuswith two beam paths and sets of lenses that converge to a common spotfor exposure or etching or doping and the two particle types performtheir functions essentially simultaneously.

For further example, resists exist that are polymerized optically usingultraviolet light. The fourth preferred embodiment could be varied suchthat an initial pattern produced using positron annihilation of certainpolymer bonds frustrates a blanket development of the thus-formed latentimage by a flood of ultraviolet light.

The invention provides the advantage of low secondary electrongeneration rate for resist exposure.

What is claimed is:
 1. A method of lithography comprising the stepof:(a) exposing a resist with annihilative particles whereinantiparticles are annihilated by electrons or nucleons in said resist toform a mask.
 2. The method of claim 1, wherein:(a) said particles have akinetic energy on the order of 1 eV.
 3. The method of claim 1,wherein:(a) said particles are positrons.
 4. The method of claim 1,wherein:(a) said resist is PMMA.
 5. The method of claim 1, furthercomprising the step of:(a) exposing said resist to a second type ofparticles which differ from said annihilative particles.
 6. The methodof claim 5, wherein:(a) said exposure by said annihilative particlesforms a latent image which deters exposure of said resist by said secondtype of particles.
 7. The method of claim 1, wherein:(a) saidannihilative particles selectively expose said resist by scanning avariable-intensity beam of said annihilative particles across saidresist.
 8. The method of claim 7, wherein:(a) said scanning by said beamof annihilative particles includes multiple scans over areas of saidresist with the dose of each of said scans insufficient to expose saidresist but with the cumulative dose of all of said multiple scanssufficient to expose said resist.
 9. The method of claim 1, wherein:(a)said annihilative particles selectively expose said resist by passingsaid annihilative particles through a mask.
 10. A method of lithographycomprising the step of:(a) etching a material with annihilativeparticles wherein antiparticles are annihilated by electrons or nucleonsin said material such that an etched pattern is obtained.
 11. The methodof claim 10, wherein:(a) said particles have a kinetic energy on theorder of 1 eV.
 12. The method of claim 10, wherein:(a) said particlesare selected from the group consisting of anti-protons, anti-neutrons,and mesons.
 13. The method of claim 10, wherein:(a) said material is asemiconductor.
 14. The method of claim 10, wherein:(a) said annihilativeparticles selectively etch said material by scanning avariable-intensity beam of said annihilative particles across saidmaterial.
 15. The method of claim 14, wherein:(a) said scanning by saidbeam of annihilative particles includes multiple scans over areas ofsaid material with the dose of each of said scans insufficient to etchsaid material but with the cumulative dose of all of said multiple scanssufficient to etch said material.
 16. The method of claim 10,wherein:(a) said annihilative particles selectively etch said materialby passing said annihilative particles through a mask.